1. Field of the Invention
The present invention relates to a passive optical network (PON) device for signal transmission between an optical line terminal (OLT) of a telephone office or a base station terminal and an optical network terminal (ONT) or an optical network unit (ONU) of a subscriber terminal in an optical subscriber network, and more particularly, to a time division multiplex (TDM)/wavelength division multiple access (WDMA) PON device in which data to be transmitted to each subscriber is multiplexed using a TDM scheme for transmission for downstream signals destined for an ONT/ONU from an OLT and signals to be transmitted from each subscriber are separately transmitted using a WDMA scheme for upstream signals destined for an OLT from an ONT/ONU.
2. Description of the Related Art
FIG. 1 is a block diagram illustrating a conventional passive optical network (PON). As illustrated in FIG. 1, in the conventional PON, an optical line terminal (OLT) 110 located in a telephone office or in a base station or a housing complex transmits a broadcast signal, a data communication signal, and a voice call signal to a subscriber over a single optical fiber. The signals sent through the single optical fiber are split by an optical splitter 120 through optical fibers connected to subscribers and are delivered to 16, 32, 64, or 128 ONTs 130 or ONUs or more. The ONT 130 or ONU converts an optical signal delivered to a subscriber terminal into an electrical signal, selects a signal dedicated_to the ONT 130 among transmitted signals, and then transmits the separated signal to the subscriber device. For signal transmission to subscribers, the PON uses a time division multiplex (TDM) scheme or a wavelength division multiplex (WDM) scheme. The TDM scheme transmits signals in different time slots for each subscribers. The WDM scheme transmits signals using different wavelengths for each subscribers.
FIG. 2 is a block diagram illustrating a conventional TDM PON. As illustrated in FIG. 2, the conventional TDM PON uses a TDM scheme in which a time division multiplexer 210 converts a plurality of downstream signals destined for subscribers into a high-speed single signal, an optical transmitter 220 modulates the converted signal into a single optical signal for transmission to the subscribers, and once the optical signal is transmitted to all the subscribers by an optical power splitter 240, each of the subscribers converts the optical signal input to an ONT into an electrical signal by using an own optical transceiver 250 and selects and receives only a downstream signal corresponding to the subscriber. For each upstream signal coming from each subscriber, a time division multiple access (TDMA) scheme is used in which the optical transceiver 250 of the subscriber transmits a signal within a time slot allocated to the subscriber to arrange the upstream signal at a corresponding point of time.
The TDM PON has been implemented using various methods such as ATM-PON, BPON, GPON, and EPON. At present, technologies capable of 1 Gb/s transmission are commonly used and PONs capable of 10 Gb/s transmission are under standardization.
In the TDM scheme, an OLT can reduce production costs compared to a wavelength division multiplex (WDM) scheme by exchanging a signal with all subscribers using the single optical transmitter 220 and an optical receiver 230. In other words, while a base station terminal requires N optical transceivers 250 for N subscribers in the WDM scheme, a base station terminal requires only one optical transmitter 220 and only one optical receiver 230 in the TDM scheme. Moreover, a broadcast or multicast signal destined for all subscribers can be transmitted to all the subscribers through a single packet in the TDM scheme. However, a transmission band is reduced because subscribers share upstream/downstream transmission bands of the optical transmitter 220 and the optical receiver 230, and a complex circuit is required for the optical receiver 230 for an upstream signal because the OLT has to receive packets of different amplitudes and phases in a burst mode format from different subscribers.
FIG. 3 is a block diagram illustrating a conventional WDM PON. As illustrated in FIG. 3, the conventional WDM PON uses, for downstream signals destined for subscribers, the WDM scheme in which the downstream signals are converted into optical signals using optical transmitters 310 which respectively have different wavelengths, the optical signals which respectively have different wavelengths are input to a single optical fiber by a wavelength division multiplexer 320 for transmission to the subscribers, a signal for each of the subscribers is separated using a wavelength splitter 340. The conventional WDM PON uses, for upstream signals transmitted from subscribers, the wavelength division multiple access (WDMA) scheme in which the upstream signals are transmitted to an OLT using optical transceivers 350 which respectively have different wavelengths.
The wavelength splitter 340 functions for downstream signals as a wavelength division demultiplexer which outputs optical signals of a plurality of wavelengths input through a port connected to the OLT to a port connected to each subscriber for each wavelength and functions for upstream signals as a wavelength division multiplexer which multiplexes an optical signal input through a port connected to each subscriber and outputs the multiplexed optical signal to a single port for transmission to the OLT.
A WDM PON is capable of 100 Mb/s-level transmission per wavelength at its early stage, and now has an increased transmission speed of up to 1 Gb/s per wavelength. Although the WDM PON may use different wavelengths for an upstream signal and a downstream signal for each subscriber, a remodulation scheme for generating an upstream optical signal by re-using a downstream optical signal as an optical source is also used as an approach for reducing costs by reducing the number of optical sources. As an example of the remodulation scheme, a method of remodulating a downstream optical signal modulated at a high speed of 10 Gbit/s to generate an upstream optical signal modulated at a low speed of 1 Gbit/s is used in the paper titled “Demonstration of Data Remodulation for Upstream Traffic in WDM Access Networks Using Injection-Locked FP Laser as Modulator” published by L. Y. Chan, et. al, in the Optical Fiber Communication Conference and Exhibit in 2001.
According to the WDM scheme, a subscriber can transmit a signal up to the transmission bands of the optical transmitter 310 and of an optical receiver 360 of the OLT because the optical transmitter 310 and the optical receiver 360 are allocated to each subscriber at a base station terminal. Furthermore, a circuit of the optical receiver 360 for an upstream signal is simplified because a packet transmitted from each subscriber to the OLT is in a continuous mode format. However, production costs increase because the numbers of optical transmitters 310 and optical receivers 360 of the OLT are the same as the number of subscribers, and the OLT has to repetitively transmit a downstream signal such as a broadcast signal at the same time to the subscribers because an optical splitter cannot perform transmission in a broadcast or multicast manner.